Methods of Forming Field Effect Transistors, Pluralities of Field Effect Transistors, and DRAM Circuitry Comprising a Plurality of Individual Memory Cells

ABSTRACT

A method of forming a field effect transistor includes forming trench isolation material within a semiconductor substrate and on opposing sides of a semiconductor material channel region along a length of the channel region. The trench isolation material is formed to comprise opposing insulative projections extending toward one another partially under the channel region along the channel length and with semiconductor material being received over the projections. The trench isolation material is etched to expose opposing sides of the semiconductor material along the channel length. The exposed opposing sides of the semiconductor material are etched along the channel length to form a channel fin projecting upwardly relative to the projections. A gate is formed over a top and opposing sides of the fin along the channel length. Other methods and structures are disclosed.

TECHNICAL FIELD

Embodiments disclosed herein pertain to methods of forming field effecttransistors, to pluralities of field effect transistors, and to DRAMcircuitry comprising a plurality of individual memory cells.

BACKGROUND OF THE INVENTION

Field effect transistors are devices commonly used in the fabrication ofintegrated circuitry. Such devices conventionally comprise a pair ofconductive source/drain regions having a semiconductive channel regiontherebetween. A conductive gate is received operably proximate thechannel region, and is separated therefrom by a dielectric material.Application of suitable voltage to the gate causes current to flow fromone of the source/drain regions to the other through the channel region,accordingly operating as a switch depending upon voltage application tothe gate.

Integrated circuitry fabrication technology continues to strive to makesmaller and denser circuits, with the corresponding size of individualdevices, of course, shrinking in the process. As the size of fieldeffect transistors gets smaller and the length of the channels betweenthe source/drain regions shortens, complex channel profiles have beendeveloped to achieve desired “on” threshold voltages and to alleviateundesired short channel effects. Such profiles for the channel regionscan include gating the channel region from multiple sides. One examplesuch device is a FinFET. Such structures are built onsemiconductor-on-insulator substrates in which the semiconductormaterial (typically silicon) is etched into a “fin”-like shaped channelbody of the transistor, with the conductive gate wrapping up and overthe “fin”.

“Fin”-shaped channel body regions have also been proposed in bulksemiconductor processing in addition to semiconductor-on-insulatorprocessing. Etching of the semiconductor material to produce the typicalvertically-extending channel fins can create shoulder areas ofsemiconductor material adjacent the base of the fins. Such areas canresult in undesired parasitic capacitance as the conductive gate is alsotypically received over these shoulder semiconductor material areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross section of a substrate fragment atcommencement of processing according to an embodiment of the invention,and taken through line 1-1 in FIG. 2.

FIG. 2 is a diagrammatic top plan view of the FIG. 1 substrate fragment.

FIG. 3 is a view of the FIG. 1 substrate fragment at a processing stepsubsequent to that shown by FIG. 1, and taken through line 3-3 in FIG.4.

FIG. 4 is diagrammatic top plan view of the FIG. 3 substrate fragment.

FIG. 5 is a view of the FIG. 3 substrate fragment at a processing stepsubsequent to that shown by FIG. 3.

FIG. 6 is a view of the FIG. 5 substrate fragment at a processing stepsubsequent to that shown by FIG. 5.

FIG. 7 is a view of the FIG. 6 substrate fragment at a processing stepsubsequent to that shown by FIG. 6.

FIG. 8 is a view of the FIG. 7 substrate fragment at a processing stepsubsequent to that shown by FIG. 7.

FIG. 9 is a view of the FIG. 8 substrate fragment at a processing stepsubsequent to that shown by FIG. 8.

FIG. 10 is a view of the FIG. 9 substrate fragment at a processing stepsubsequent to that shown by FIG. 9, and taken through line 10-10 in FIG.11.

FIG. 11 is a diagrammatic top plan view of the FIG. 10 substratefragment.

FIG. 12 is a view of the FIG. 10 substrate fragment at a processing stepsubsequent to that shown by FIG. 10.

FIG. 13 is a view of the FIG. 12 substrate fragment at a processing stepsubsequent to that shown by FIG. 12.

FIG. 14 is a view of the FIG. 13 substrate fragment at a processing stepsubsequent to that shown by FIG. 13.

FIG. 15 is a view of the FIG. 14 substrate fragment at a processing stepsubsequent to that shown by FIG. 14, and taken through line 15-15 inFIG. 16.

FIG. 16 is a diagrammatic top plan view of the FIG. 15 substratefragment.

FIG. 17 is a view of the FIG. 15 substrate fragment at a processing stepsubsequent to that shown by FIG. 15, and taken through line 17-17 inFIG. 18.

FIG. 18 is a diagrammatic top plan view of the FIG. 17 substratefragment.

FIG. 19 is a diagrammatic cross section of another embodiment substratefragment.

FIG. 20 is a view of the FIG. 19 substrate fragment at a processing stepsubsequent to that shown by FIG. 19.

FIG. 21 is a diagrammatic cross section of yet another embodimentsubstrate fragment.

FIG. 22 is a view of the FIG. 21 substrate fragment at a processing stepsubsequent to that shown by FIG. 21.

FIG. 23 is a view of the FIG. 22 substrate fragment at a processing stepsubsequent to that shown by FIG. 22.

FIG. 24 is a schematic representation of DRAM circuitry.

DETAILED DESCRIPTION

Example embodiments of the invention are described in connection withFIGS. 1-24. Referring initially to FIGS. 1 and 2, a semiconductorsubstrate is indicated generally with reference numeral 10. In thecontext of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductor material, including, but not limited to, bulksemiconductor materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductormaterial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. Substrate 10 is depicted as comprising bulk semiconductorsubstrate material 12, for example monocrystalline silicon. Substrate 12may, of course, comprise a different substrate, for example includingsemiconductor-on-insulator substrates and other substrates whetherexisting or yet-to-be developed.

A field trench isolation mask 15 has been formed and patterned oversubstrate material 12. In the depicted embodiment, such comprises a padoxide layer 14 having a silicon nitride-comprising layer 13 formedthereover. Much of the material beneath layers 14 and 13 of field trenchisolation mask 15 will constitute active area, while much of the exposedregion of mask 15 will constitute trench isolation.

Referring to FIGS. 3 and 4, a pair of trenches 16 has been etched withinsemiconductor substrate 10 into semiconductor material 12. An exampleetch depth for trenches 16 is from 800 to 1,000 Angstroms. For purposesof the continuing discussion, semiconductor material 12 may beconsidered as comprising a semiconductor material channel region 18comprising opposing sides 20 and 22 extending along a length “L” of thechannel region 18. Accordingly, trenches 16 are formed on opposing sides20, 22 of semiconductor material channel region 18 along channel lengthL. Substrate 10 would typically, of course, comprise more masked regions15, and a series of such trenches 16 would likely be etched oversubstrate 10. An example dry anisotropic etching chemistry to producethe FIGS. 3 and 4 construction includes a combination of HBr and Cl₂.

Referring to FIG. 5, trenches 16 have been lined with one or moresuitable masking materials 24, and which has been subsequentlyanisotropically etched to expose a semiconductor material base 26 ofsubstrate material 12. An example material 24 is silicon nitride formedby chemical vapor deposition and/or by plasma or other nitridation ofsemiconductor material 12. An example lateral thickness of material 24is from 60 Angstroms to 90 Angstroms. Accordingly, such provide but oneexample manner by which trenches 16 can be formed to have linedsidewalls and an exposed semiconductor material base 26.

Referring to FIG. 6, semiconductor material bases 26 (not shown) havebeen substantially isotropically etched through effective to form abulbous lower portion 27 of each trench 16. Each of bulbous lowerportions 27 comprises projections 28, 29 extending laterally outwardrelative to the lined trench sidewalls referred to above. One projectionof each bulbous lower portion 27 opposes and extends towards aprojection of the other bulbous lower portion, with the projections thathave been designated with numeral 28 being shown as constituting suchexample opposing projections. For purposes of the continuing discussion,bulbous lower portions 27 may be considered as comprising respectivefloors 30. Where semiconductor material 12 comprises monocrystallinesilicon, an example isotropic etching chemistry to produce the depictedbulbous lower portions includes a dry etching chemistry using HBr andNF₃. An example added depth to trenches 16 beyond the depth shown by theFIG. 3 etch is from 800 to 1,000 Angstroms.

Referring to FIG. 7, substantially anisotropic etching has beenconducted through floors 30 of bulbous lower portions 27 to extend pairof trenches 16 deeper within semiconductor substrate 10. An exampleadded depth for the depicted lower stem portions of such trenches isfrom 500 to 1,000 Angstroms. Most desirably, the etch chemistry andparameters are switched back to anisotropic in situ.

Referring to FIG. 8, trenches 16 have been lined with one or moresuitable materials 32, for example one or more layers of silicon dioxideand/or silicon nitride. Such might be deposited by one or both ofchemical vapor deposition and/or thermal/plasma nitridation and/oroxidation of the sidewalls of the depicted trenches. An examplethickness for layer 32 is from 50 to 150 Angstroms.

Referring to FIG. 9, one or more insulative materials 34 have beendeposited effective to fill remaining volume of trenches 16 withinsulative material. Material 34 is also depicted as being planarizedback at least to the outer portion of silicon nitride layer 13.Alternatively and by way of example only, trench isolation masking layer13 (and also perhaps layer 14) may be removed from the substrate priorto deposition of insulative material 34. Regardless, an example material34 is high plasma density deposited silicon dioxide.

Such provides but one example method of forming trench isolationmaterial 34 within a semiconductor substrate 12 and on opposing sides20, 22 of a semiconductor material channel region 18 along a length L ofthe channel region. Trench isolation material 34/32 can be considered ascomprising opposing insulative projections 36 which extend toward oneanother along channel length L, and insulative projections 38. In oneembodiment, semiconductor material 12 of substrate 10 is receivedover/atop insulative projections 36, as shown. In one embodiment,insulative projections 36 are received partially under channel region18, as shown.

As referred to above, trench isolation masking material 13 may beremoved from the substrate prior to or after the formation of trenchisolation material 34. Regardless, preferably substrate 10 at this pointwill be patterned for ultimate desired formation of fin channel featureswhile protecting the cell contact, bit contact, and field trenchisolation regions of the structure. Such might be accomplished in anynumber of manners, with FIGS. 10 and 11 illustrating but one embodimentof such masking and patterning. FIGS. 10 and 11 depict materials 13 and14 having been removed, and insulative material 34 having been etchedback. One or more masking materials 40 have been deposited and patternedprimarily for the fabrication of fin-channel regions. Material 40patterned over channel regions 18 will not necessarily be patterned toconform to the outline of channel regions 18 (as shown). Further, suchmay be patterned to essentially cover all (not shown) of thesemiconductor material between trench isolation material 34/32 in theFIG. 10 cross-section. Alternatively and by way of example only, and aswill be subsequently described in connection with another embodiment,all of such semiconductor material in the FIG. 10 cross-section betweentrench isolation material 34 may be outwardly exposed, and thereby notmasked by material 40. An example preferred material 40 is siliconnitride deposited to an example thickness range of from 600 to 1,200Angstroms.

Referring to FIG. 12, trench isolation material 34 has been etched toexpose opposing sides 41 of semiconductor material 12 along channellength L. Such etching might be isotropic, anisotropic, or a combinationof one or more of anisotropic and isotropic etching steps. Where trenchisolation material 34 comprises high density plasma deposited silicondioxide, an example anisotropic dry etching chemistry comprises acombination of C₄F₆, C₄F₈, O₂, He, and Ar, whereas an example isotropicwet etching chemistry comprises a buffered aqueous HF solution. Where alining 24 remains from the example preferred FIG. 5 processing, andwhere such comprises silicon nitride, such is also etched (as shown) andan example silicon nitride etching chemistry to expose semiconductormaterial sidewalls 41 comprises a combination of CH₂F₂ and O₂.

FIG. 12 illustrates the etching of trench isolation material 34 beingconducted at least elevationally to opposing insulative projections 36,which is preferred. FIG. 13 illustrates an example of continuing theFIG. 12 etching in a dry, substantially anisotropic manner into trenchisolation material 34 which is laterally adjacent the trench insulativematerial 34/32 of opposing insulative projections 36. In one embodimentand as shown, such etching of trench isolation material 34/32 isdepicted as not being into any insulative material 34/32 within theopposing insulative projections 36, although other embodiments are ofcourse contemplated, for example as will be described below. Further inone embodiment and as depicted in FIG. 13, opposing insulativeprojections 36 can be considered as having some elevational thickness“T” having an elevational mid-point “M”, and having floors “F”. Etchingof trench isolation material 34, as shown in FIG. 13, has been at leastto mid-point M of elevational thickness T, and is precisely thereat. Theetching of trench isolation 34 and 32, however, is desirably notconducted all the way to floors F.

Referring to FIG. 14, exposed opposing sides 41 (not shown due to theirremoval) of semiconductor material 12 have been etched along channellength L to form a channel fin 45. In the depicted example FIG. 14embodiment, such is projecting upwardly, preferably relative to opposinginsulative projections 36. For purposes of the continuing discussion,semiconductor material 12 along channel length L can be considered ashaving a top 46, with such top 46 being masked during etching of theexposed opposing sides of semiconductor material 12 to form channel fin45, and with such masking occurring by way of example only from material40. Another embodiment is described below whereby example top 46 isunmasked during the semiconductor material etching to form channel fin45. Regardless, etching of semiconductor material 12 to form projectingchannel fin 45 may desirably be conducted in a substantially anisotropicmanner, with an example of an etching chemistry to produce to the FIG.14 construction comprising starting with a combination of CF₄ and He,and finishing with HBr.

Referring to FIGS. 15 and 16, an example of subsequent processing isshown whereby masking material 40 has been removed. Outlines 48 areshown that comprise transistor source/drain regions that have or will befabricated and that connect with a fin channel region 45.

Referring to FIGS. 17 and 18, a gate 52 has been formed over a top andopposing sides 20, 22 of fin channel region 45 along channel length L.Such is depicted as being formed by forming a gate dielectric layer 54,followed by the deposition of one or more conductive layers 56(including one or more conductively doped semiconductor layers), andpatterning of at least conductive material 56 into line-shapedconfigurations 52, for example as shown in FIG. 18. Source/drain dopingand/or construction may be subsequently finalized, or may have beenessentially completed previously to form source/drains 48. For example,FIG. 18 depicts two transistors 51 and 53 having been fabricated, andwhich by way of example share a source/drain region 48 between thedepicted gate lines 52.

The above-described embodiment masked the top of the semiconductormaterial along the channel length during etching of the exposed opposingsides of the semiconductor material to form the channel fin. By way ofexample only, another embodiment is shown in FIGS. 19 and 20 withrespect to a substrate fragment 10 a. Like numerals from thefirst-described embodiment have been utilized where appropriate, withdifferences being indicated with the suffix “a”. FIG. 19 is analogous tothe FIG. 13 substrate depiction; however, where masking material 40 ofFIG. 13 has been removed from/is not provided over what will be the finchannel region. Further, a greater quantity of semiconductor material 12has been provided above opposing insulative projections 36.

Referring to FIG. 20, exposed opposing sides of semiconductor material12 have been etched along channel length L to form an upwardlyprojecting channel fin 45 a. Accordingly in the depicted FIGS. 19 and 20example, the top of material 12 along channel length L is unmaskedduring the etching of the exposed opposing sides of semiconductormaterial 12 to form the channel fin, and the etching of such topdesirably occurs during the etching of the exposed opposing sides toform the channel fin. A combination of isotropic and anisotropic etchesmight be conducted in lieu of the foregoing. Regardless, gates (notshown) may be fabricated subsequently, analogous to that shown in FIGS.17 and 18.

Another embodiment is shown in FIGS. 21-23 with respect to a substratefragment 10 b. Like numerals from the first-described embodiment havebeen utilized where appropriate, with differences being indicated withthe suffix “b”. FIG. 21 essentially depicts processing subsequent to orcontinuing of that shown by the first embodiment substrate of FIG. 12.FIG. 13 depicted the etching of trench isolation material 34 in a mannerwhich was not into any insulative material within opposing insulativeprojections 36. Etching however may also, of course, occur intoinsulative projections 36 in connection with the above-identifiedsubstrates 10 and 10 a embodiments. By way of example only, FIG. 21depicts an embodiment wherein at least some of trench isolation material34/32 is etched from opposing insulative projections 36 to formprojections 36 b and 38 b. FIG. 21 illustrates substantially isotropicetching of trench isolation material 34/32 and within projections 36 toelevational mid-point M. An example isotropic etching chemistry toremove material 34 includes an aqueous buffered HF solution. Anisotropic etching chemistry to remove material 24 and 32, where suchcomprise silicon nitride, includes a combination of CH₂F₂ and O₂.

FIG. 22 depicts subsequent etching of the exposed opposing sides ofsemiconductor material 12 along channel length L to form an upwardlyprojecting channel fin 45 b. FIG. 23 depicts subsequent processing forthe fabrication of a gate 52 b, including conductive material 56 b andgate dielectric 54 b.

The above substrates 10 and 10 a provide embodiments whereby insulativematerial 34/32 within each of opposing projections 36 is at leastpartially received under upwardly projecting fin 45. Further, thesubstrates 10 and 10 a embodiments depict substrates having insulativeprojection inner surfaces 95 (FIGS. 17 and 20) extending along thelength of the channel which are convexly curved relative to the finthickness transverse the channel length. The FIG. 22 embodiment depictsone example field effect transistor wherein none of insulative material34/32 within each of opposing projections 36 b in the finishedconstruction is received under upwardly projecting channel fin 45 b.

The above-described processing is particularly desirable wherein theetching of some of the trench isolation material occurs from opposinginsulative projections prior to etching the exposed opposing sides ofthe semiconductor material to form the channel fin. Embodiments of theinvention also contemplate conducting at least some of the etching ofthe trench isolation material from the opposing insulative projectioncommensurate with the etching of the exposed opposing sides of thesemiconductor material to form the channel fin. By way of example only,a single substantially anisotropic etching chemistry may be utilized todirectly go from the FIG. 10 depiction to produce the FIG. 22construction.

Some embodiments of the invention, of course, encompass methods offorming one or more field effect transistors by the above-describedmethods. Some embodiments of the invention also contemplate a pluralityof field effect transistors independent of the method of fabrication. Byway of example only, one embodiment contemplates a plurality of fieldeffect transistors wherein individual of such transistors comprise asemiconductor substrate comprising a pair of source/drain regions havinga fin channel region received therebetween. The fin channel regioncomprises a channel length extending between the pair of source/drainregions, opposing channel sides extending along the length of thechannel region, and a top extending along the length of the channelregion. The fin channel region has a maximum thickness transverse thechannel length.

A gate is received over the fin channel top and the channel sides alongthe channel length. Insulative material is received immediately beneaththe fin channel region extending along the channel length, and extendsonly partially across the fin channel maximum thickness transverse thechannel length. The insulative material includes opposing portionsprojecting inwardly toward one another under the fin channel regionrelative to the fin channel maximum thickness along the channel length.By way of example only, an individual of such field effect transistorsis shown with respect to the embodiments exemplified by FIGS. 17, 18 and20 above. Desirable sizes and materials of construction andconfigurations may otherwise be as described above.

An embodiment of the invention encompasses a plurality of field effecttransistors wherein individual of such transistors comprise a bulksemiconductor substrate comprising a pair of source/drain regions havinga fin channel region received therebetween. The fin channel regioncomprises a channel length extending between the pair of source/drainregions, opposing channel sides extending along the length of thechannel region, and a top extending along the length of the channelregion.

A gate is received over the fin channel top and the channel sides alongthe channel length. Trench isolation is received within the bulksemiconductor substrate elevationally lower than the fin channel regionand extends along the opposing channel sides along the channel length.The trench isolation in cross-section transverse the channel lengthcomprises a lower trench stem and upper transverse projections extendingfrom the stem transversely towards and elevationally lower than the finchannel. Each of the above embodiments depict such an example individualfield effect transistor channel region, wherein the lower portion of thetrench etched below the bulbous portion can be considered as a lowertrench stem having upper transverse projections encompassed byprojections 36/36 b.

Embodiments of the invention also encompass DRAM circuitry comprising aplurality of individual memory cells. Individual of the memory cellscomprise a field effect transistor having a pair of source/drainregions, a capacitor connected with one of the source/drain regions, anda bit line contact connected with another of the source/drain regions.For example, FIG. 24 depicts an example such DRAM memory cell 75encompassing a transistor 53 (i.e., transistor 53 of FIG. 18). Acapacitor 70 is connected with one of source/drain regions 48 and a bitline contact 80 connected with another of source/drain regions 48. Forexample, bit line contact 80 would connect with source/drain region 48shown in FIG. 18 between the depicted gate lines 52 of transistor 73with a bit line, and the lower-depicted source/drain region 48 oftransistor 53 in FIG. 18 would connect with an appropriate capacitor 70.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of forming a field effect transistor, comprising: formingtrench isolation material on opposing sides of a semiconductor materialchannel region along a length of the channel region, the trenchisolation material being formed to comprise upper sidewalls and opposinginsulative projections below the upper sidewalls that extend laterallyrelative to the upper sidewalls toward one another, the insulativeprojections being received partially elevationally under the channelregion along the channel length and with semiconductor material beingreceived elevationally over the projections; exposing opposing sides ofthe semiconductor material along the channel length by removing some ofthe trench isolation material; removing some of the semiconductormaterial from the exposed opposing sides along the channel length toform a channel fin projecting upwardly relative to the projections; andforming a gate over a top and opposing sides of the fin along thechannel length.
 2. The method of claim 1 wherein forming the trenchisolation material comprises forming insulative projection innersurfaces extending along the length of the channel which are convexlycurved relative to fin thickness transverse the channel length.
 3. Themethod of claim 1 wherein the semiconductor material along the channellength has a top, the top being masked during the removing ofsemiconductor material from the exposed opposing sides to form thechannel fin.
 4. (canceled)
 5. The method of claim 1 wherein the removingof the trench isolation material does not remove any insulative materialwithin the projections.
 6. The method of claim 1 wherein the etching ofthe trench isolation material is into insulative material which islaterally adjacent insulative material of the opposing insulativeprojections.
 7. (canceled)
 8. The method of claim 6 wherein the removingof the trench isolation material is into insulative material within theopposing insulative projections.
 9. (canceled)
 10. The method of claim 6comprising forming the opposing insulative projections to have anelevational thickness, the removing of the trench isolation being atleast to a midpoint of the elevational thickness.
 11. The method ofclaim 10 wherein the opposing insulative projections are formed to havefloors, the removing of the trench isolation not being to said floors.12. A method of forming a field effect transistor, comprising: formingtrench isolation material on opposing sides of a semiconductor materialchannel region along a length of the channel region, the trenchisolation material being formed to comprise upper sidewalls and opposinginsulative projections below the upper sidewalls that extend laterallyrelative to the upper sidewalls toward one another along the channellength and with semiconductor material being received elevationally overthe projections; exposing opposing sides of the semiconductor materialalong the channel length by removing some of the trench isolationmaterial and removing some of the trench isolation material from theopposing insulative projections; removing some of the semiconductormaterial from the exposed opposing sides along the channel length toform a channel fin; and forming a gate over a top and opposing sides ofthe fin along the channel length.
 13. The method of claim 12 wherein thesemiconductor material that is removed comprises bulk monocrystallinesilicon.
 14. The method of claim 12 wherein the semiconductor materialthat is removed comprises semiconductor material of asemiconductor-on-insulator substrate.
 15. The method of claim 12 whereinthe removing of some of the trench isolation material from the opposinginsulative projections occurs prior to removing the exposed opposingsides of the semiconductor material to form the channel fin.
 16. Themethod of claim 12 wherein at least some of the removing of the trenchisolation material from the opposing insulative projections occurscommensurate with the removing of the exposed opposing sides of thesemiconductor material to form the channel fin.
 17. (canceled)
 18. Amethod of forming a field effect transistor, comprising: etching a pairof trenches within a semiconductor substrate on opposing sides of asemiconductor material channel region along a length of the channelregion, the trenches comprising lined sidewalls and an exposedsemiconductor material base; etching the semiconductor material baseseffective to form a bulbous lower portion of each trench, each of thebulbous lower portions comprising projections extending laterallyoutward relative to the lined sidewalls, a projection of each bulbouslower portion opposing and extending toward a projection of the otherbulbous lower portion; etching through floors of the bulbous lowerportions to extend the pair of trenches deeper within the semiconductorsubstrate; after extending the pair of trenches, filling remainingvolume of the trenches with insulative material; after said filling,etching the insulative material to expose opposing sides of thesemiconductor material along the channel length; etching the exposedopposing sides of the semiconductor material along the channel lengthforming an upwardly projecting channel fin; and forming a gate over atop and opposing sides of the fin along the channel length.
 19. Themethod of claim 18 comprising providing the insulative material withineach of the opposing projections to be at least partially received underthe upwardly projecting channel fin.
 20. The method of claim 18comprising providing none of the insulative material within each of theopposing projections to be received under the upwardly projectingchannel fin.
 21. A field effect transistor comprising: a semiconductorsubstrate comprising a pair of source/drain regions having a fin channelregion received therebetween; the fin channel region comprising achannel length extending between the pair of source/drain regions,opposing channel sides extending along the length of the channel region,and a top extending along the length of the channel region; the finchannel region having a maximum thickness transverse the channel length;a gate received over the fin channel top and the fin channel sides alongthe channel length; and insulative material received immediately beneaththe fin channel region and beneath the gate, such insulative materialbeneath the fin channel region and beneath the gate extending along allof the channel length and extending only partially across the finchannel maximum thickness transverse the channel length, such insulativematerial beneath the fin channel region and beneath the gate includingopposing portions projecting inwardly toward one another beneath the finchannel region relative to the fin channel region maximum thicknessalong all of the channel length and beneath the gate along all of thechannel length.
 22. A field effect transistor comprising: a bulksemiconductor substrate comprising a pair of source/drain regions havinga fin channel region received therebetween; the fin channel regioncomprising a channel length extending between the pair of source/drainregions, opposing channel sides extending along the length of thechannel region, and a top extending along the length of the channelregion; a gate received over the fin channel top and the fin channelsides along the channel length; and trench isolation received within thebulk semiconductor substrate elevationally lower than the fin channelregion and beneath the gate, said trench isolation extending along theopposing channel sides along all of the channel length, the trenchisolation in cross section transverse the channel length comprising alower trench stem and upper transverse projections, the upper transverseprojections extending from the stem transversally towards andelevationally lower than the fin channel and the gate. 23-24. (canceled)25. The field effect transistor of claim 21 wherein the inwardlyprojecting portions respectively comprise an inwardly projecting convexsurface immediately beneath the fin channel region.
 26. The field effecttransistor of claim 21 wherein the inwardly projecting portionsrespectively comprise an arcuate surface extending continuously fromimmediately beneath the fin channel region to parallel a line transversethe channel length.
 27. A method of forming a field effect transistor,comprising: forming trench isolation material on opposing sides of asemiconductor material channel region along a length of the channelregion, the trench isolation material being formed to comprise opposinginsulative projections extending toward one another partially under thechannel region along the channel length and with semiconductor materialbeing received over the projections; exposing opposing sides of thesemiconductor material along the channel length by removing some of thetrench isolation material; removing some of the semiconductor materialfrom the exposed opposing sides along the channel length to form achannel fin projecting upwardly relative to the projections, thesemiconductor material along the channel length having a top, the topbeing unmasked during the etching of the exposed opposing sides of thesemiconductor material to form the channel fin, and further comprisingetching the top during the etching of the exposed opposing sides of thesemiconductor material to form the channel fin; and forming a gate overa top and opposing sides of the fin along the channel length.